Pound for pound Chum Salmon, when fresh from the ocean. Although I got a 72 pound Kenai King that was probably my best salmon fight ever. Big sturgeon are a lot of fun too, but they are much bigger than any other fresh water fish.
In the salt, pound for pound either a bonefish, permit, or tuna. I've never caught a big bonefish but the 3 to 5 pounders I caught in acsencion bay were smokers on fly gear. Tuna just flat out put the wood to you.